Duplicate entries in jac_structure

[MaxenceGollier]

I have an issue when using PDENLPModels.jl.

When I call

(rows,cols) = jac_structure(nlp)

on some PDENLPModel, the tuple (rows,cols) can have duplicate entries which creates bugs on some linear algebra packages (like QRMumps.jl).
I think it would be easier to add a condition in the jac_structure function to prevent this.

I have a minimal working example if needed which is just the test/problems/controleslaticmembrane.jl example with n=20.

[tmigot]

Hi @MaxenceGollier ! Sorry for the late reply.

What do you mean by "it would be easier to add a condition in the jac_structure function to prevent this"?

It is not trivial to do avoid duplicate. When we collect all the contributions from the mesh to the Jacobian, we would have to iterate over all collected values to see if it is already there. True, it saves memory, but it costs time too.

We could add a post-treatment function that given the tuple (rows, cols, vals) return another tuple without duplicate, though.

[MaxenceGollier]

Ok, I was expecting it to be too costful.
I don't think adding a post-treatment function here will be useful for me.
Maybe it would be useful to add some kind of warning in the documentation though ?

[tmigot]

Ok, why not. Any suggestion about where we could put it?
I don't think the NLPModel API documentation suggest that there are no duplicate entries, but you are right that it is worth mentioning it.

[MaxenceGollier]

Do you have any ideas where we could add this information ? Ideally, I would add some kind of
"Warning: jac_structure(nlp) can have duplicate entries which may cause errors if directly sent to LinearAlgebra libraries."
I think adding a "@warn" is perhaps a bit too much, I don't know what do you think ?